E‐cigarette and cannabis use and co‐use patterns and dependence trajectories among young adults in the United States

BACKGROUND AND AIMS: Co-use of cannabis and e-cigarettes for nicotine is increasing among younger adults in the United States (US). This study aimed to determine (1) the patterns of cannabis and e-cigarette use and co-use over time and (2) the trajectories of e-cigarette and cannabis dependence symptoms across latent classes representing different patterns of use and co-use. DESIGN: Prospective cohort study, with data collected every six months at six time points. SETTING: Hawaii, USA. PARTICIPANTS: N = 1305 individuals [mean age = 25.0 (standard deviation = 2.2); 52% women], who reported past-30-day cannabis and/or e-cigarette use at any of the six data collection time points between March 2021 and February 2024. MEASUREMENTS: For the repeated measures latent class analysis (RMLCA): past-30-day cannabis use, e-cigarette use and co-use (any past-30-day use of both cannabis and e-cigarettes for nicotine). RMLCA covariates: demographic variables and cigarette smoking. For the latent growth curve analyses: e-cigarette dependence [Penn State E-Cigarette Dependence Index (PSEDI), range: 0-20] and cannabis dependence symptoms [Cannabis Abuse Screening Test (CAST), range: 0-24]. FINDINGS: Four latent classes were identified: individuals who, across time points, consistently showed (1) low probabilities (≤ 0.17) of using e-cigarettes or cannabis, including co-use (44% of the sample); and (2) high probabilities (> 0.50) of primarily e-cigarettes-only use (18%); (3) primarily cannabis-only use (20%); and (4) primarily co-use (18%). For co-use class, the average trajectory for cannabis dependence remained flat [mean linear slope (LS) = 0.01 (standard error [SE] = 0.03), P > 0.05] whereas that for e-cigarette dependence increased over time [LS = 0.15 (0.03), P < 0.0001] before showing a slight decline [mean quadratic slope (QS) = -0.02 (0.007), P < 0.01]. For e-cigarette-only use class, e-cigarette dependence increased over time [LS = 0.91 (0.05), P < 0.0001], before showing a slight decline [QS = -0.02 (0.006), P < 0.0001]. The levels of e-cigarette dependence were higher for e-cigarette-only class than the co-use class across all time points. For cannabis-only class, the average trajectory of cannabis dependence symptoms did not change over time [LS = 0.03 (0.03), P > 0.05]. The levels of cannabis dependence symptoms were higher for the co-use class than the cannabis-only class across all time points. CONCLUSIONS: Among young adults who reported past-30-day cannabis and/or e-cigarette use, on average, e-cigarette dependence increased over time for both e-cigarette-only and co-use classes before showing slight declines. Compared with the co-use class, the e-cigarette-only class tended to show higher levels of e-cigarette dependence across all time points. Cannabis use dependence did not change statistically significantly for either co-use or cannabis-only classes.